Stress controlling mounting structures for printed circuit boards

ABSTRACT

A printed circuit board having a plurality of traces and pads printed  then. Slots or openings of various geometric shapes are formed in the board to provide flexible mounting members. The pads are mounted on the mounting members. Surface-mounted electrical components have their terminals soldered to the pads. The flexibility of the mounting members permits small movements of the soldered joints in response to thermal expansions, vibrations, etc. to control potentionally damaging stresses.

The Government has rights in this invention pursuant to Contract No.DAAK10-84-C-0079 awarded by the Department of the Army.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to printed circuit boards and, moreparticularly, to mounting structures that control stresses betweenelectrical components and conductive surfaces on printed circuit boards.

2. Description of Related Art

In the field of electronic circuit fabrication, it has been the generalpractice to employ printed circuit assemblies wherein much of thecircuit wiring and the circuit components are mounted on a common base.In general, a printed circuit usually comprises a relatively rigid baseon which a pattern of printed wires is formed in some predeterminedconfiguration. The printed wiring is usually etched from a previouslydeposited layer of copper cladding. The printed wiring generallyincludes narrow conductive strips called "circuit traces" and broadconductive surfaces called "pads". The traces and pads providepoint-to-point electrical connections for the separately manufacturedelectrical components, such as resistors, capacitors, transistors, etc.The components are usually mounted on the pads by soldering or otherprocess to produce a conductive contact between the component'sterminals and the pads.

Modern-day circuit fabricators often prefer to use printed circuitboards to other structures because of the neatness and miniaturizationmade possible by their use. There are additional benefits to be realizedfrom the use of circuit boards: Low lead inductance, improved physicalstability of the components and interconnecting leads, and goodrepeatability of the basic layout of a given circuit. The repeatabilityfactor makes the use of circuit boards ideal for mass productiontechniques.

There are a number of alternative construction techniques that areemployed for mounting the components on the circuit boards. One simplemethod is to use leaded components. In this method, the component'sleads are first bent into an appropriate shape and the component is thenmounted on the board by soldering the bent leads to spaced pads on theboard. The lead-pad interconnection provides some mechanical stabilityin addition to the necessary electrical coupling of the component to theprinted circuit.

Another mounting method, an important space-saving technique, involvesthe use of surface-mounted components. These components, often referredto as chips, are used widely in UHF and microwave circuits. Thesurface-mounted components are usually small blocks having conductivesurfaces at either end that act as terminals. When used on printedcircuit boards, the conductive surfaces are soldered directly to thepads.

Although the various mounting alternatives have served the purpose, theyhave not proved entirely satisfactory under all conditions of servicefor the reason that considerable difficulty with component mounting hasbeen experienced. Specifically, circuit failures occur frequently due tostress-induced fractures that develop at the soldered interconnectionsbetween the components and the pads. Leaded components and adhesivebonding are the main technologies currently used for stress control ofsmall electrical components to avoid such failures. However, leadedcomponents do not lend themselves to dense packaging or very highproduction rates. They are also more expensive than surface-mounteddevices. Adhesive bonding is not an attractive high-rate process, is asecondary operation adding recurring costs and is limited by theadhesive properties.

As such, cost and competition are driving the electronics industry tosurface mount components so as to more densely pack the components intoa smaller space. Also, low-cost circuit boards fabricated from plasticmaterials by injection molding are now being adopted by the electronicsindustry to lower costs. Those skilled in these arts recognize that oneof the most critical problem confronting designers of such circuits isachieving sufficient physical stability of the componentinterconnections on the boards to avoid stress-induced failures due tosuch causes as thermal expansions, vibrational fatigue, and the like.

It is, therefore, a primary object of the present invention to controlstresses induced between printed circuit boards and the componentsmounted thereon.

SUMMARY OF THE INVENTION

The general purpose of this invention is to provide a printed circuitboard which embraces all of the advantages of similarly employed circuitboards and possesses none of the problems associated with thermal andfatigue incompatability between the components and the board. To attainthis, the present invention contemplates a circuit board having meansfor controlling stresses induced between the board and componentsmounted thereon. In the present invention, stresses are controlled bymolding or cutting geometric forms to provide flexible mounting memberson which the susceptible components are mounted. The flexibility of themounting members on the board then sustains the stresses and not thecomponents. The stress-controlling structures are most cost effectivewhen applied to the relatively new technology of molded printed circuitboards, but may be used with all types of boards and components.

In more detail, the invention comprises a printed circuit assembly thatincludes a board, a plurality of conductors mounted on the surface ofthe board, first and second spaced conductive surfaces on whichelectrical components are mounted and at least one of the surfacesmounted on a mounting member flexibly joined to the board.

The exact nature of this invention as well as other objects andadvantages thereof will be readily apparent from consideration of thefollowing specification relating to the annexed drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a preferred embodiment of the invention.

FIG. 2 is a sectional view taken on the line 2--2 of FIG. 1.

FIG. 3 is a plan view of another embodiment of the invention.

FIG. 4 is a plan view of a further alternate embodiment of theinvention.

FIG. 5 is a pictoral view of still another alternate embodiment of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference charactersdesignate like or corresponding parts throughout the several views,there is shown in FIGS. 1 and 2 a section of a printed circuit board 11having an upper trace 12 and a lower trace 13. Trace 12 is connected toa pad 14. Trace 13 is connected to a pad 15. Traces 12, 13 and pads 14,15 may be formed in the conventional manner by etching of a copper cladsurface on the board 11.

An H-shaped slot 16 in the board 11 forms two cantilever beams 21 and 22on which the pads 14 and 15 are mounted. An electronic component 17,having conductive terminals 18 and 19, is mounted directly to the pads14, 15 by soldering terminals 18, 19 to pads 14, 15, respectively.

The slot 16 may be readily formed during the molding process or it maybe cut at a later time before or after the etching process. The size andshape of the beams 21, 22, and their material characteristic willprimarily determine their degree of flexibility. It is noted that thebeams 21, 22 will usually be of the same thickness as board 11, but theymay be of a different thickness to produce a particular amount offlexibility. Those skilled in these arts may readily determine theappropriate dimensions for the beams 21, 22 to produce a desiredflexibility. Of course, beams 21, 22 may each be of the same or adifferent size. For some situations one of the beams, say beam 21, maybe eliminated.

The beams 21, 22 generally provide flexibility in a directionperpendicular to the board 11 as indicated by the arrows in FIG. 2. Incontrast, the embodiment of FIG. 3 provides flexibility in the plane ofthe board 11, generally in the direction of the arrows shown in FIG. 3.In the FIG. 3 embodiment, four U-shaped slots 36, 37, 38, 39, formed inboard 11, define two broad surfaces 30, 31, each joined to the main bodyof board 11 by pairs of flexible, narrow arms 41, 42 and 43, 44,respectively. The electronic component 17 is mounted on the board 11 bysoldering the conductive terminals 18, 19 directly to soldering pads 34,35, mounted on the surfaces 30, 31, respectively. A circuit trace 32 isjoined to pad 34 and extends to the main body of board 11 via arm 42. Acircuit trace 33 is joined to pad 35 and extends to the main body ofboard 11 via arm 43.

The size and shape of the U-shaped slots 36-39 may be chosen by thefabricator to provide an appropriate amount of flexibility as required.In general, the narrow arms 41-44 will have sufficient flexibility sothat the surfaces 30, 31 can move with respect to each other to relieveany stresses caused by thermal expansions and contractions, mechanicalvibrations and the like. As in the embodiment of FIGS. 1 and 2, thematerial properties of the circuit board 11 must be considered whenchoosing the sizes and shapes of the slots 36-39.

FIG. 4 shows still another configuration that will control stressesdirected both perpendicular to and parallel to the plane of the board11. The board 11 has a W-shaped slot 56 which forms a pair of elongatedcantilever beams 57 and 58. A soldering pad 54 is mounted near the freeend of beam 54 while a soldering pad 55 is mounted near the free end ofbeam 58. The electronic component 17 has its conductive terminals 18, 19soldered directly to the pads 54, 55, respectively. Circuit traces 52,53 extend from the main body of board 11 along the surface of beams 57,58 to join pads 54, 55, respectively.

Because of the narrow profile of the beams 57, 58, they will move withrespect to each other and with respect to the main body of board 11 inthe plane of the board 11 as well as in a direction perpendicular to theplane of board 11. Of course, the beams 57, 58 will undergo resultantdisplacements in directions that are the vector sum of the displacementsjust described.

To summarize, the various embodiments of FIGS. 1-4 illustrate techniquesfor controlling stresses by providing flexible mounting structurescapable of movements with respect to each other and with respect to themain body of a relatively rigid circuit board 11. As such, when properlydesigned to produce the necessary flexibility, these mounting structurescan protect against failures due to thermal expansions and contractions,shock-wave vibrations, steady-state vibrations, and otherstress-inducing forces. It is noted that the composite structures madeup of a component 17 and its associated mounting structure will have anatural vibrating frequency. As such, the circuit board designer maydesign the mounting structure to effectively tune this natural frequencyto a desired value. For example, the natural frequency may be tuned toproduce a critical mismatch between it and some mechanical,stress-inducing vibration that may be expected to appear in the board11. As such the mechanical vibrations will be damped due to the mismatchthereby protecting the soldered joints.

In many cases it may be necessary to mount a sensitive electroniccomponent on a single surface. For example, a crystal resonator in somecases must be mounted on a common surface having a pair of conductorsthat are electrically coupled to a extended surfaces of the crystal.FIG. 5 depicts such a situation.

In FIG. 5, the circuit board 11 includes a pair of U-shaped slots 60, 61that form a pair of cantilever beams 63, 64. A quartz crystal 65 ismounted on the beam 63. A pair of circuit traces 67, 68, printed on theboard 11, extend onto the beam 63 to provide a conductive surface ontowhich the crystal 65 is connected. A second pair of circuit traces 69,70 are shown mounted on the surface of board 11. Traces 69, 70 extendonto the beam 64 to provide conductive surfaces onto which an electricalcomponent 71 (shown in phantom) may be mounted.

As indicated earlier, the structures shown in FIG. 5 can be designed ortuned to damp out potential vibrations. In accordance with principleswell known to those skilled in these arts, the material characteristicsof the board 11, the size and shape of the beams 63, 64, as well as theweight, size and placement of the crystal 65 on the beam 63 are allfactors that may be controlled to produce a desired natural frequencyfor the composite structure.

The geometric features of the various mounting structures shown in FIGS.1-5 may be readily added to the tool used for molding the printedcircuit boards 11. In other words, the boards 11 may be molded with theappropriately shaped and sized openings 16, 36-39, 56, 60 and 61. Whenmolding techniques are used to produce the boards 11, the addition ofthese mounting features will be nonrecurring once the design of theproduct is fixed. Of course, the addition of such mounting features tostandard printed circuit boards, after the boards are formed, bypunching or cutting, for example, will also have the same mechanicalbenefits; however, the cost advantages will usually not be assignificant.

Various other modifications and similar embodiments are contemplated andmay obviously be resorted to by those skilled in the art withoutdeparting from the spirit and scope of the invention, as hereinafterdefined by the appended claims, as only preferred embodiments thereofhave been disclosed.

The foregoing disclosure and drawings are merely illustrative of theprinciples of this invention and are not to be interpreted in a limitingsense. It is to be understood that the invention should not be limitedto the exact details of construction shown and described because obviousmodifications will occur to a person skilled in the art.

What is claimed is:
 1. A printed circuit assembly comprising:a board; atleast one mounting member flexibly attached to said board; a pattern ofprinted wires mounted on the surface of the board; first and secondspaced conductive surfaces connected to said wires and at least one ofwhich is mounted on said mounting member; and an electronic componentfixed to each of said conductive surfaces.
 2. The circuit board of claim1 wherein said mounting member is at least partially surrounded by anopening formed in said board.
 3. The circuit board of claim 2 whereinsaid board has a flat, planar body and said mounting member lies in theplane of said body.
 4. The circuit board of claim 3 wherein saidmounting member and said board are formed of a common material.
 5. Thecircuit board of claim 4 wherein said component is mounted on saidboard.
 6. The circuit board of claim 4 wherein said member is formed asa cantilevered beam flexibly attached to said board.
 7. The circuitboard of claim 4 wherein said member is formed as a broad surface withat least two narrow flexible arms extending between said broad surfaceand said body.
 8. The circuit board of claim 6 wherein said cantileveredbeam is flexibly mounted for movement in the plane of said board and ina direction transverse to the plane of said board.
 9. The circuit boardof claim 6 wherein said first and second spaced conductive surfaces aremounted on said at least one mounting member.
 10. A printed circuitassembly comprising:a board; first and second mounting members, eachflexibly attached to said board; a pattern of printed wires mounted onthe board; first and second conductive surfaces connected to said wiresand each surface mounted on a different one of said members; and anelectronic component having terminals each fixed to one of saidsurfaces.
 11. The circuit board of claim 10 wherein each said member isat least partially surrounded by an opening in said board.
 12. Thecircuit board of claim 11 wherein said board has a flat, planar body andsaid mounting members lie in the plane of said body.
 13. The circuitboard of claim 12 wherein said mounting members and said board areformed of a common material.
 14. The circuit board of claim 13 whereinsaid members are formed as cantilevered beams flexibly attached to saidboard.
 15. The circuit board of claim 14 wherein said members eachinclude a broad surface with two flexible arms extending between saidbroad surface and said body.
 16. The circuit board of claim 14 whereinsaid beams are flexibly mounted for movement in the plane of said boardand in a direction transverse to the plane of said board.
 17. Thecircuit board of claim 14 wherein said opening is an H-shaped slotformed in said board that surrounds the free edges of each of saidcantilevered beams.
 18. The circuit board of claim 15 wherein saidopening includes a pair of U-shaped slots fored in said board.
 19. Thecircuit board of claim 16 wherein said opening is a W-shaped slot formedin said board.
 20. The circuit board of claim 12 wherein said componentis surface mounted on said body.